Microelectrode studies of chemical reactions
Neumann's integral theorem has been used to establish exact analytical solutions for chronoamperometric experiments and for the steady state limiting current for CE and catalytic EC mechanisms. To convert these solutions into a useful form, FORTRAN programs have been written for the necessary numerical calculations. The analytical response of microdiscs, for chronoamperometry as well as the steady state for CE and EC' reactions is presented. A different theoretical approach, as well as digital simulations, are used for the interpretation of steady state limiting currents for EC' reactions, in particular, silver(II)-substrate coupled reactions. The radius dependence of the ratio between the kinetically and the diffusion controlled currents has been calculated for various kinetic schemes. The anodic oxidations of Cr(III), water and of Mn(II) in the presence of catalytic quantities of Ag(I), were investigated using Pt microdiscs with radii between 0.3 and 62.5 m. Because of the enhanced rate of diffusion to microelectrodes, kinetic currents are observed for the Ag(I) mediated oxidation of Cr(III). It is shown that there is good agreement between the experimental i_k/i_d= f(a) plots and those computed assuming a mechanism where the rate determining step is electron transfer from Cr(III) to Ag(II). The rate constant was determined as 9x10^6 mol^-1 cm^3 s^-1. The Ag(II)-Mn(II) reaction must be substantially faster since in most conditions the measured current is determined by the diffusion of Mn(II) to the surface. As expected but in complete contrast, the Ag(II) water reaction is too slow to observe kinetic currents at microelectrodes.